1. Field of the Invention
This invention relates in general to sensing the torque and weight applied to a drill string and, in particular, to sensing the torque and weight applied to the drill string at mud line and sub mud line levels.
2. Brief Description of Related Art
In subsea drilling operations, a drilling vessel generally floats over an area that is to be drilled. The drilling vessel then runs a drilling riser that extends from the surface drilling vessel to a wellhead located at the sea floor. The drilling riser serves as the lifeline between the vessel and the wellhead as most drilling operations are performed through the drilling riser. As the devices are needed for the well, such as casing hangers, bridging hangers, seals, wear bushings, and the like, they pass from the surface of the vessel on a drill string through the riser, through the wellhead and into the wellbore. Weight and rotation are used to place and actuate these devices. Because of this, it is important to know with some specificity the weight and torque applied to the device in the subsea environment to know that the device has reached the appropriate position in the wellbore and properly actuated. Typically, this is accomplished by measuring the applied torque and weight at the drilling vessel.
Because drilling vessels float over the subsea wellhead, they are subject to the effects of ocean currents and winds. Ocean currents and winds will push drilling vessels such that they do not remain completely stationary over the wellhead, despite attempts to anchor them to the sea floor. In addition, the riser itself is subject to movement due to ocean currents. Because of this, the riser will not remain truly vertical between the wellhead and the drilling vessel. Instead, the riser will “curve” in response to the position of the vessel in relation to the wellhead and the effects of the current on the unanchored riser sections extending between the ends anchored at the drilling vessel and at the wellhead. As locations in deeper water are explored, the problem becomes exacerbated.
As the riser curves, the drill string passing through the riser will contact the riser rather than remaining suspended between the riser walls. At the locations where the drill string contacts the riser wall, the drill string becomes anchored, and transmits some of the operational weight and torque applied by the drilling vessel to the drill string from the drill string to the riser. Thus, the actual torque and weight applied to the device in the wellbore is less than the total torque and weight applied at the drilling vessel. Because the devices rely on the appropriate weight and torque to land, set, and test in the appropriate position in the wellbore, loss of torque and weight due to anchoring of the drill string against the riser may mean that operators at the drilling vessel are not properly testing, setting, or landing the devices because they are basing their actions on the torque and weight measurements taken at the drilling vessel. To ensure that appropriate torque and weight are applied to land, set, and test the devices, measurements of applied torque and weight at the device location are necessary.
One prior art method for sensing that appropriate weight and torque have been applied to set a downhole consumable involves the use of specially grooved casings at the landing locations of the downhole consumable. Proximity sensors are then incorporated into the consumable or alternatively in a separate tool inline with the consumable. The proximity sensors are tripped when the grooves on the special casing are proximate to the sensors. The proximity sensors then generate an acoustic signal that is received at the platform and interpreted as a landing of the tool. Unfortunately, these devices require the use of specialty tools and specialty casings in order to appropriately generate a setting/landing signal. In addition, the devices are unable to provide information about the applied weight and torque at the consumable that may indicate whether the rig and well are out of position relative to one another or that the drill string has become anchored.
Another prior art method for sensing the weight and torque at a downhole location involves the use of suspended strain gauge sensors to measure and record weight and torque at downhole locations. However, these sensors are used not to determine what is happening in real time, but instead to determine frictional losses during drilling, prior to the setting of any downhole consumables. The data and calculations from these devices are studied and used to conduct drilling operations at similar locations and formation types. They do not provide real time feedback to an operator during the landing, setting, and testing of wellhead consumables. Therefore, there is a need for a method and apparatus for sensing weight and torque in a mud line while landing, setting, and testing subsea wellhead devices.